If it is an infection, your options are limited. You may be able to clear it by increasing salinity a bit. You should be able to find articles online about what salinity to use, but pathogens tend to be a bit more sensitive about that than the frog.

If you are willing to risk it, Tetracycline is available for fish OTC in water soluble capsules(Marketed as Fishcycline). It is broad spectrume and may work. Tetracycline though tends to cause skin damage to amphibians.